Wednesday, September 24, 2014

Fall Temperature Drop Off

As Richard noted yesterday, temperatures are dropping in the Interior. This should not come as a surprise to anyone who lives in most of Alaska. As the sub-solar point drops south of the equator (Autumnal Equinox) there is less and less solar energy to prop up the temperatures. October is traditionally known as the time of greatest temperature decline but that is not the case everywhere.

Let us begin with the amount of temperature change. I looked at the 1981-2010 NCDC normals for all stations in Alaska and identified the maximum decline for the daily normal temperature during any 31-day period. The results of that analysis are shown in Figure 1. Approximately 25% of Alaska observes a 31-day temperate drop in the fall/winter of 23°F or greater and half of the state sees a drop of at least 20°F. The statewide "winner" is actually the Woodsmoke Cooperative station near Fairbanks with a seasonal drop of 27.9°F. Central is next with 27.2°F followed by Chicken with 26.6°F. The smallest 31-day change in the Fairbanks area is Keystone Ridge with a value of 19.3°F. At the other end of the spectrum is Adak in the far western Aleutian Islands. Their seasonal temperature drop (31-days) is only 6.0°F. Dutch Harbor, Shemya, St. George, and St. Paul are all at, or under, 7.0°F.

Figure 1. Largest 31-day decline in normal temperatures throughout the year based on NCDC normals.

Anecdotal, we think of October 1st as the beginning of this seasonal drop off but that is not necessarily the case. Figure 2 shows the first day of the greatest 31-day temperature decline for all of Alaska based on stations with NCDC published normal temperatures for 1981-2010. As it turns out, the October 1st line runs right through the Fairbanks area. However, nearly half of Alaska does not get going on their fall temperature swoon until the first week of October. In southwestern Alaska, it begins in the middle to later portions of September.

Figure 2. Starting date of the largest 31-day decline in normal temperatures throughout the year depicted in Figure 1.

2 comments:

Brian, it's interesting to visualize this - thanks. It's a little surprising to me that the eastern North Slope sees a drop almost as rapid as the interior - but I suppose once ice cover sets in, the maritime influence is greatly reduced and temperatures can go into free fall.